The invention concerns a measuring sensor for the capacitative measurement of liquids by means of electrically insulated electrodes and more particularly concerns measuring sensors suitable for use in enamelled steel vessels and the like.
An example of a known measuring sensor of this type is described in U.S. Pat. No. 4,021,707 wherein it is disclosed that a platinum tape may be embedded in the enamel layer of a dip tube along the length of the dip tube, so that the filling level may be measured with the aid of that part of the electric field lines, which cross through a liquid on their path from the electrode to the metallic material. A second electrode may be embedded in the enamel layer at the bottom of the dip tube, so that the capacity between the second electrode and the steel body of the dip tube may be used for compensating the capacitance between the first electrode and the steel body, which is a function of the dielectric constant and/or the conductivity of the respective vessel filling material.
Such measuring sensors have proven to work well under operating conditions, since, due to the possible compensation, a sufficiently accurate continuous measurement can be carried out even in those cases where subsequent dielectric constants or conductivities of liquid filling materials differ. An accurate measurement of the filling level or monitoring of a reaction is not possible, however, if the liquid in the vessel is not homogeneous, for instance if the dielectric constant in the lower part of the vessel differs considerably from the dielectric constant of the liquid in the upper part of the vessel. It is therefore the purpose of this invention to improve a measuring sensor of the above referenced type in such a manner that continuous measurement of the filling level in partial areas is possible in inhomogeneous liquids, particularly in the case of different dielectric constants, or that interface levels, interface measurements, and other controls of the homogeneity of liquids with respect to the dielectric constant value can be carried out.